1. Technical Field
The present invention relates to a semiconductor integrated circuit, and more particularly, to a duty cycle correction apparatus provided to a semiconductor integrated circuit.
2. Related Art
A conventional semiconductor integrated circuit such as a DDR SDRAM (double data rate synchronous dynamic random access memory), processes data using both the rising edge and the falling edge of a clock signal to increase an operation speed. Therefore, if the ratio between the high level interval and the low level interval of a clock signal, that is, a duty ratio does not correspond to 50:50, operation efficiency is likely to deteriorate. However, in fact, the clock signal employed in a semiconductor integrated circuit is difficult to have a precise duty ratio due to various factors in the mounting circumstances of the semiconductor integrated circuit, such as noise, etc. Thus, in order to improve operation efficiency, the semiconductor integrated circuit includes a duty cycle correction apparatus for correcting the duty ratio of a clock signal.
A duty cycle correction apparatus is provided in a DLL (delay locked loop) circuit and is implemented in such a way as to correct the duty cycles of the pair of clock signals output from the DLL circuit. However, even when the duty cycle correction operation for the clock signals is performed at the output terminal of the DLL circuit, a phenomenon is likely to occur, in which the duty ratios of the clock signals are distorted due to delay factors in a transmission line. Also, in order to transmit the pair of clock signals, the transmission line must be implemented by a form of a pair of lines, whereby the area and power utilization efficiency of the semiconductor integrated circuit is likely to deteriorate. As a consequence, the conventional duty cycle correction apparatus has drawbacks in that it should be provided in the DLL circuit and thus the signal distortion and the deterioration of area and power utilization efficiency are likely to be caused by the transmission lines.